Curable resins represented by phenol resin, unsaturated polyester resin, epoxy resin and the like have been used widely in various fields due to excellent heat resistance, mechanical strength, dimensional accuracy and the like. Particularly, the epoxy resin has been used widely in construction materials, electric and electronic materials, adhesives, fiber reinforced composites and the like due to many merits such as excellent mechanical strength, electrical insulation, heat resistance, adhesive property and the like. However, there is a problem that the molded product obtained from the epoxy resin shows very brittle property due to small fracture toughness.
As a method for reinforcing toughness by adding a modifier to an epoxy resin, a method for adding a rubber component in the epoxy resin composition is known. As a method for adding a rubber component, there are known to be a method for adding a reactive liquid rubber (CTBN and the like) or a nitrile rubber, a method for mixing a core-shell polymer with an epoxy resin and the like (for example, Patent Document 1). However, since the reactive liquid rubber is subjected to phase separation at the time of curing after dissolving in the epoxy resin, there are problems that given modified effects are not obtained and the reproducibility of the quality is deteriorated by the change of morphologies of the cured products obtained according to the differences of the kinds of the epoxy resin compounded and the curing conditions. In addition, since the rubber component is partially dissolved and left in the epoxy resin phase after curing, there is a problem that the quality of the epoxy resin product is decreased and the like due to the decrease of the elastic modulus and glass transition temperature of the cured product.
In addition, a method for adding a core-shell polymer in an epoxy resin can control the decrease of the glass transition temperature. However, there is a problem that the core-shell polymer is separated by easily precipitating or floating a mixed core-shell polymer because the core-shell polymer is commercially available in powder of several ten micron meters to hundred micron meters as aggregate of the primary particles, and the core-shell polymer must be finely powdered to less than 10 μm, heated and agitated at the temperature of 50 to 200° C., and carefully mixed by high shear device, heat roll, intermixer, kneader, triple roll mill and the like in the case of mixing with the epoxy resin.
On the other hand, Patent Document 2 discloses a method for obtaining a resin composition in which the rubber polymer particles are favorably dispersed and impurities are small, by contacting a mixture of an aqueous latex containing the rubber polymer particles and an organic solvent having the partial solubility to water, with water to prepare the aggregate of the rubber polymer particles, separating the aqueous phase from the mixture of the aggregate and the aqueous phase to prepare the aggregate of the rubber polymer particle having impurities in a small amount, adding an organic solvent to the aggregate to prepare a dispersion, mixing the dispersion with a polymerizable organic compound having a reactive group such as an epoxy resin, and distilling a volatile component from the mixture. However, the shell layer of the core-shell polymer disclosed in this method has higher glass transition temperature (hereinafter referred to as Tg) than room temperature, and the composition contains a resin component having high Tg in which effects for improving the impact resistance of the epoxy resin is insufficient, so that there is room for further improvement.
In addition, Patent Documents 3 and 4 disclose a composition containing a core-shell polymer and a thermosetting resin, and the shell part of the core-shell polymer has Tg of 20° C. or less and a hydroxyl group. However, this core-shell polymer increases the viscosity of the composition, the handling becomes difficult, and the impact resistance is not exhibited in some cases, so that there is room for further improvement.